All three pathogenic Yersinia species, Y. pestis, Y. pseudotuberculosis and Y. enterocolitica, the causative agents of plague, mesenteric adenitis and enterocolitis, respectively, promote their own survival and subsequently cause disease by inhibiting the host innate immune response. This response is dependent on an intact type III secretion system that translocates at least six proteins from the bacteria into the mammalian host cells. Despite its discovery over fifteen years ago and its essential role in virulence, little is known about the function of one of these proteins, YopM, in these processes. YopM is a member of the growing family of bacterial leucine-rich repeat proteins. YopM, as well as two other members of this protein family, localize to the mammalian nucleus during infection. Since conventional means have not yet revealed a function for this interesting protein, the newly developed system for studying bacterial virulence proteins in the yeast Saccharomyces cerevisiae will be employed. Notably, when expressed in yeast, YopM localizes to the nucleus and confers an inhibitory growth phenotype. Since nuclear transport is highly conserved between yeast and mammals, these phenotypes will be exploited to characterize the regions of YopM and the cellular pathways required for YopM nuclear localization in yeast. A YopM expression system will also be developed in mammalian cell lines to confirm that YopM nuclear signals are conserved among eukaryotes and to study direct effects of YopM function. YopM mutants that no longer localize to the mammalian nucleus will be tested for virulence in the mouse model system for plague. Lastly, the well developed functional genomic and genetic tools of S. cerevisiae will be employed to identify potential roles for YopM in Yersinia pathogenesis. Together, these studies should shed light on many areas of research including (1) the role of nuclear localization in bacterial pathogenesis, (2) mechanisms of nuclear localization, (3) insights into the role of YopM and this growing family of bacterial LRR proteins in pathogenesis, and (4) hypotheses for the role of YopM in pathogenesis to be tested in a directed manner in higher eukaryotes.